1. Field of the Invention
This invention relates to computer graphics systems and, more particularly, to methods and apparatus for deriving from a frame buffer signals originating from both interlaced and non-interlaced formats for presentation on either an interlaced or non-interlaced output display monitor.
2. History of the Prior Art
It is the vision of many that in the near future a person sitting at a personal computer will be able to call up on the screen of the computer monitor information from a myriad of sources. For example, it is expected that a person will be able to hear telephone and radio communications, view television or recorded motion pictures, play stereo recordings of music, and operate computer graphical and text programs. It is also expected that all of these operations will be possible at the same time so that, for example, a television program may be displayed in one window while a computer graphics program is running in another window or computer graphics material may be displayed as an overlay on the television program.
It has long been recognized by those skilled in the art that is much easier to visualize the wonderful results that one would like than to reach those results, especially where the results require the combining of television (video) signals with computer graphics signals on the same output monitor. The crux of the problem is that, although both types of signals are electrical, they arrive in entirely different formats for their two purposes. The television signals are analog signals. It is desirable that these analog television signals be converted to digital representations for presentation on a computer monitor so that they may be moved and manipulated and the television window in which they appear may be resized. Moreover, in the United States the video signals are presented in accordance with the NTSC standard in an interlace pattern of a first field of 240 lines followed one-sixtieth of a second later by a second field of 240 lines to form a complete picture. In Europe, a PAL standard interlace pattern consists of a first field of 288 lines followed one-fiftieth of a second later by a second field of 288 lines to form a complete picture. Obviously, other standards are possible. From this point on, only the U.S. standard will be used in the discussion though other standards should be considered to be included. This interlaced method of presentation allows a less expensive monitor to present pictures which are entirely acceptable for television. However, such a monitor is not acceptable for computer graphics where much more detail must be displayed and manipulated. Consequently, a typical computer display presents 480 lines of data in a non-interlaced mode.
Thus, the data from these two different sources of two different types, interlaced and non-interlaced, must somehow be presented in a form which can be handled by a personal computer. The usual method suggested is to convert the video data and place it in a first frame buffer, place the computer data in a second frame buffer, and somehow switch between the two frame buffers in presenting the data to an output monitor. Another more difficult method would be to somehow place all of the data from the two sources in the same frame buffer; again, the problem remains of how to switch the data to an output monitor.
The reason that this is a problem is two-fold. First, the data is stored in one or two frame buffers in interlaced form if it came from a video source and in non-interlaced form if it is computer data. The visionary also expects to be able to present the output on either an interlaced television type monitor or a computer monitor of some sort. Thus, it is desirable that the operator be able to combine video data from an interlaced source (hereinafter called interlaced data) and non-interlaced computer data and display both forms of data on both interlaced and non-interlaced monitors without disconcerting visual effects.
Presenting interlaced data on a monitor designed to display interlaced signals is not a problem; such a monitor simply takes the 240 lines of interlaced information available in a first field and presents it on the 240 lines available on the monitor. Then it follows this with the next 240 lines which are interleaved and offset in time to make up the complete picture.
However, presenting the non-interlaced data on a monitor designed to display interlaced signals is a greater problem. Non-interlaced data has 480 lines which are not offset in time. If every other line is displayed to make up a first field and then the alternate 240 lines are displayed to make up an interleaved second field, the fact that the computer graphics is of higher resolution causes flickering which is disconcerting to the viewer. Consequently, the lines of the non-interlaced computer display must somehow be adapted to appear correct to the viewer when presented on an interlaced output monitor.
In a similar manner, presenting non-interlaced data on a monitor designed to display non-interlaced signals is not a problem; for such a monitor simply takes the 480 lines of non-interlaced information available and presents it all on the monitor. However, presenting the interlaced data on a monitor designed to display non-interlaced signals is a greater problem. Interlaced data has only 240 lines per field followed by a second 240 lines which are offset in time. If both sets of 240 lines are displayed together to make up a non-interleaved frame of 480 lines, the fact that lines which are time offset are presented together provides a picture which is incorrect when motion occurs. Consequently, the lines of the interlaced video display must somehow be adapted to appear correct to the viewer when presented on an non-interlaced output monitor.
Thus, it is clear that whether a monitor is designed to present either interlaced or non-interlaced data, you must somehow change some of the data if both types are to be displayed on the same monitor.